package org.tony.nio.demo.channels;

import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.net.SocketException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.DatagramChannel;

public class TimeServer
{

	private static final int	DEFAULT_TIME_PORT	= 37;
	private static final long	DIFF_1900			= 2208988800L;
	protected DatagramChannel	channel;

	public TimeServer(int port) throws Exception
	{
		this.channel = DatagramChannel.open();
		this.channel.socket().bind(new InetSocketAddress(port));
		System.out.println("Listening on port " + port + " for time requests");
	}

	public void listen() throws Exception
	{
		// Allocate a buffer to hold a long value
		ByteBuffer longBuffer = ByteBuffer.allocate(8);
		// Assure big-endian (network) byte order
		longBuffer.order(ByteOrder.BIG_ENDIAN);
		// Zero the whole buffer to be sure
		longBuffer.putLong(0, 0);
		// Position to first byte of the low-order 32 bits
		longBuffer.position(4);
		// Slice the buffer; gives view of the low-order 32 bits
		ByteBuffer buffer = longBuffer.slice();
		while (true)
		{
			buffer.clear();
			SocketAddress sa = this.channel.receive(buffer);
			if (sa == null)
			{
				continue; // defensive programming
			}
			// Ignore content of received datagram per RFC 868
			System.out.println("Time request from " + sa);
			buffer.clear(); // sets pos/limit correctly
			// Set 64-bit value; slice buffer sees low 32 bits
			longBuffer.putLong(0, (System.currentTimeMillis() / 1000) + DIFF_1900);
			this.channel.send(buffer, sa);
		}

	}

	public static void main(String[] args) throws Exception
	{
		int port = DEFAULT_TIME_PORT;
		if (args.length > 0)
		{
			port = Integer.parseInt(args[0]);
		}
		try
		{
			TimeServer server = new TimeServer(port);
			server.listen();
		}
		catch (SocketException e)
		{
			System.out.println("Can't bind to port " + port + ", try a different one");
		}
	}

}
